The invention relates to a monopulse FM-CW radar system comprising a transmitter, an antenna array of a transmitting antenna and at least two receiving antennas. Mixers supply the subtraction beat signal between the transmitted signal and each echo signal received by each receiving antenna after having been reflected from a target. The system also includes means for processing the beat signals for effecting detection of the target bearing as well as detection of the range and/or relative velocity of the target within a range gate of a predetermined width.
The invention is used to the best advantage in simplified radar equipment, preferably air-borne equipment, and must have the properties of being sturdy, compact and inexpensive. It relates more specifically to radar equipment for missiles.
Monopulse FM-CW radar systems are known and described in, for example, Radar Handbook, Skolnik, published by Mc Graw-Hill Book Company, edition of 1970, pages 21-10 to 21-30. These radars operate with one single pulse transmitted either in two orthogonal planes or in a sole plane (in plan position, for example). For the sake of simplicity of the description, it will be limited hereinafter to the last-mentioned case. A monopulse FM-CW performs angular detection (bearing) and range and/or velocity detection. The angular detection is obtained by comparing the phase of the signals received on the antenna (the description will be limited to two receiving antennas). If two antennas R.sub.1 and R.sub.2 spaced by d receive a signal coming from the direction which is at an angle .phi. with the axis of the array of receiving antennas R.sub.1 and R.sub.2, the phase shift .phi. between the signals received by the respective antennas R.sub.1 and R.sub.2 is expressed by: ##EQU1## .lambda. being the wavelength of the radiated wave. If the diagrams of the antenna are assumed to be identical, this implicates that the two fields E.sub.1 and E.sub.2 received as an echo from a target are equal and in-phase for .phi.=0. It is then possible to control the assembly R.sub.1, R.sub.2 to their correct positions, by means of the value of the difference of the two received fields, which are standardized with respect to their sum. For example: EQU E.sub.1 =Eo cos (.omega.t-.phi./2) (2) EQU E.sub.2 =Eo cos (.omega.t+.phi./2) (3) ##EQU2## or in amplitude (with an accuracy of one quadrature): ##EQU3## In practice, radars of the above-defined type render it possible to obtain from the sum and the difference of the subtractive beat signals, and more specifically by means of synchronous demodulators, a term of the form: k sin .phi. where k is a constant which can be determined.
Comparing the phases is consequently the parameter which defines the angular precision of the system. This comparison may be realized directly in high frequency, which necessitates a very good control of the phases of all the components located upstream of the differential stage (TOS of the junctions and the antennas). The simplified equipment which is the object of the invention comprises in principle no components, operating in the high-frequency range, of such a precision and of such a complex construction. This comparison may alternatively be effected after a change in frequency, which is the case for radars to which the invention applies. Then, in addition to the phase errors introduced by the high-frequency input stage, phase errors are introduced by the amplifiers and the mixers.
On the other hand, the range detection is obtained by observing the correlation signal between transmission and reception. In, for example, the case of transmitting a linearly modulated signal having a frequency deviation F during the time T, the signal is delayed on reception by the value: EQU .tau.=2 D/c (7)
c being the velocity of the electromagnetic wave and D the range of the target, and is shifted by the Doppler effect characterized by the beat frequency: EQU f.sub.d =2 v/.lambda. (8)
where v is the velocity with respect to the target or the Doppler velocity. In these circumstances, the useful subtractive beat signal has a frequency f.sub.b, such that: ##EQU4##
The value of f.sub.d can be isolated by means of a specific known processing of the beat signals, which renders it possible to known the value v and, in addition, the use of the formulae (7) and (9) enables the determination of D. This greatly simplified example shows that the range gating is performed by the equivalent of a frequency filter. Obviously, in the real case of a radar the receiving periods are in general distinct from the transmission periods to avoid the saturation phenomena of the receiver. Then a local oscillator performs the part of a reference signal, but the operation is similar.
Radars of a very simple contruction intended for proximity fuses or for use as automatic direction finders may remain simple and without critical adjustments. In the case of FM-CW radars such as defined above, the control of the phase identity during manufacture and in the time is critical.